1. Field of the Invention
The invention relates to a method for storing pulse oximetry sensor and intra-aortic balloon (IAB) catheter characteristics. More particularly, the invention relates to a pulse oximeter probe or an IAB which incorporate a memory unit to store useful parameters.
2. Description of the Prior Art
A pulse oximeter sensor (probe) is a device containing generally two light emitters of differing wavelengths and a photodetector, arranged so as to admit the photodetector to respond to light from the emitters which either diffuses through, or is scattered back by, the tissue of a patient's extremity. The signals, corresponding to the different wavelengths, are analyzed by a companion pulse oximeter instrument, which determines the oxygen saturation of the blood flowing through the tissue. As is the case for any colormetric analysis, the accuracy of the oxygen determination is dependent on the accuracy to which the emitter wavelengths are known. Since the sensors are interchangeable, and often even disposable, accurate results require that the sensors be manufactured with well controlled emitter wavelengths, or that the sensor include a means for encoding the emitter wavelength error, so that the oximeter may apply a calibration correction. Such a means of error coding and correction allows inexpensive LEDs (Light Emitting Diodes), with only loose control of wavelength, to be used as emitters, while still maintaining accurate oximeter response. During manufacture of the sensor, this coding means is arranged to reflect the wavelength error of the LEDs. In operation, the oximeter reads the coding means and applies a suitable correction, as indicated by the coding means, to the oximeter readings, thereby compensating for the LED wavelength errors.
U.S. Pat. Nos. 4,621,643 and 4,700,708 disclose a pulse oximeter which uses resistors or resistance elements, embedded in either the sensor or the sensor's electrical connector, to codify the wavelength error. The error is indicated by the ohmic value of the resistor. A microprocessor, common in all modern pulse oximeters, uses the value of the resistor to choose between a number of different correlation tables, which are necessary in calculating blood oxygenation. A number of correlation tables, each containing data corresponding to a specific wavelength of light, are stored in the microprocessor. The value of the resistor acts as a code, which is interpreted by the microprocessor, indicating the wavelength of light emitted by the specific LED which was consciously paired with the resistor in the probe during manufacture. The resistor is attached to the probe, the detachable probe portion of the pulse oximeter, rather than the oximeter itself, so that probes can be used interchangeably with any compatible pulse oximeter. During use, the resistance of the resistive element in the probe is determined and this value is used by the microprocessor to choose which wavelength correlation table to use in calculating the blood oxygenation of the patient.
During manufacture of such a prior art pulse oximeter, the LEDs are characterized for wavelength and sorted into wavelength groups. Each group is then assembled into sensors bearing a resistor, the value of which is indicative of the particular wavelength group. The process of testing and sorting the LEDs prior to sensor assembly adds considerable material handling burden and cost to the manufacturing process. A more ideal process would feature a memory means which could be adjusted or programmed after the sensor is completely assembled. This would allow the LEDs to be assembled into sensors without the need for pre-testing or sorting. At the final test of the sensor/probe, the wavelengths would be measured, and the appropriate information programmed into the memory means.
A second disadvantage of the resistor coding method is that it encodes only a single parameter, that of the wavelength error. It may be desirable to encode other parameters, such as LED intensity, date of manufacture, sensor type, etc. Additional parameters can be encoded by the use of multiple resistors, but this rapidly becomes unwieldy as the number of parameters increases. An ideal coding means would allow multiple parameters to be encoded in a single device during final test stage at the conclusion of the manufacturing process.
Similar to the pulse oximeter, many intra-aortic balloon (IAB) catheters utilize a resistor to encode operating parameters. Intra-aortic balloon (IAB) catheters are used in patients with left heart failure to augment the pumping action of the heart. The catheters, approximately 1 meter long, have an inflatable and deflatable balloon at the distal end. The catheter is typically inserted into the femoral artery and moved up the descending thoracic aorta until the distal tip of the balloon is positioned just below or distal to the left subclavian artery. A passageway for inflating and deflating the balloon extends through the catheter and is connected at its proximal end to an external pump. The IAB catheter is connected to the pump by means of a connector. An encoding resistor, similar to the one used in the pulse oximeter, is generally embedded in the connector and is used to encode the volume of the IAB balloon.
A disadvantage of the resistor coding method is that it encodes only a single parameter. It may be desirable to encode other parameters, such as IAB dead volume, error detection code, expiration date, flow restriction, helium diffusion rate, membrane thickness, serial number, and configuration. Additional parameters can be encoded by the use of multiple resistors, but this rapidly becomes unwieldy as the number of parameters increases.
While pulse oximeters and IAB catheters incorporating the resistor coding method may be suitable for the particular purpose employed, or for general use, they would not be as suitable for the purposes of the present invention as disclosed hereafter.